In a radio system using Long Term Evolution (LTE) technology, the physical cell identity—PCI—that is assigned to and transmitted by all LTE cells, is a layer 1 radio signature of which there exists 504 unique data sequences. The PCI serves as a cell signature that is easily detectable for User Equipments (UEs) connectable to the radio system. Since virtually all cellular networks deploy more than 504 cells, the PCIs will have to be reused within a network.
User Equipments use the PCI to determine and assess the relation to the serving cell and to identify neighboring cells. If two cells very close to each other use the same PCI, a terminal will have difficulties to determine if it is actually moving from one cell to another. This type of problem is here referred to as problem type 1.
The terminals in a cell typically look for neighboring cells for mobility reasons to perform handover from one cell to another. An active User equipment/terminal is normally requested to detect PCIs from neighboring cells and report any detected PCI together with some signal measure, for example the received power or the signal quality of the PCIs. If two cells in the vicinity use the same PCI, there will be difficulties in determining which of the neighbor cells a moving terminal has detected. This type of problem is here referred to as problem type 2. It is even difficult to realize that there is a problem since the ambiguity may only turn up as an increased amount of handover failures to one of the neighboring cells.
Even if the LTE standard allows for 504 different PCIs, the number of available PCIs for a cell may be substantially less than that if the operator splits the PCI space into smaller fractions. That is, an operator may choose to use one PCI range for outdoor macro cells, another range for other outdoor cells, another range for indoor systems and yet another range for home base stations. In such a scenario the number of available PCIs assignable to cells of a cellular radio network will be significantly smaller.
There have been discussions within the Third Generation Partnership Program (3GPP) to introduce an automated configuration of the Physical Cell Identity, see for example Nokia contribution R3-080376 to working group RAN3. This contribution suggests that a cell can request the PCI assignment from a neighbor, and also request a list of their neighbor cells and their corresponding PCIs over the direct evolved NodeB (eNB)-interface, X2, or via the OSS) from all its neighbors. Such an approach is illustrated to FIG. 1 where a cell requests a neighboring cell to return its neighbor list and the PCIs the neighbors use.
Traditionally, an offline planning tool would create suitable PCI values for the cells in the network and transfer these in some way to an operation support system (OSS) that in turn would assign the PCI values to the cells. The planning tool would typically distribute the PCI values in such way that the risk for a UE to run into the ambiguities described above as problem types 1 and 2 is minimized.
Using a planning tool typically causes a need for manual intervention and/or many operational steps. For example one or many of the following operations typically involve manual attention: the act of running the tool itself, considering the results, transferring the values to an OSS, assigning a PCI value to a new cell, scheduling a time for implementing PCI changes for the already existing cells, and update information storage systems about the new configuration.
It is therefore desirable to make the PCI selection and assignment process fully automatic, without any operator intervention at all or at least with less manual intervention.
Making a completely new PCI plan every time a new radio base station is introduced in the network is not desirable, since changing the PCI in a cell may have disadvantageous effects for User Equipments connected to a cell or for User Equipments in the process of moving into another cell.
Hence, there exist a need for a method and a device that enables a more effective PCI selection assigning process.